In conventional processes for heat treating and, in particular, for drying and/or fixing continuously moved fabric webs (for example in so-called tenter frames), the quantity of waste air accumulating amounts to around 10 cubic meters per kg of fabric. The quantity of waste air accumulating during drying depends upon the extent to which the air is saturated with water (an enrichment of the air with at most 100 g of water per cubic meter of air is normally selected for drying). The quantity of waste air which accumulates during heat-fixing is determined by the organic substance content of the air.
The large quantities of waste air referred to above are extremely undesirable for a number of reasons. Not only do they represent a considerable loss of heat, they also give rise to considerable problems if the recently prescribed maximum pollutant content of the waste air is not to be exceeded.
In one known process of the type referred to above (DT-OS No. 2,063,444), the quantity of waste gas continuously removed from the treatment unit is initially freed from condensible constituents present in it. Some of this waste gas is then returned to the treatment unit after reheating, whilst the rest of the waste gas is released into the atmosphere. Accordingly, this known process is essentially a means of pretreating the waste air of which the efficiency is largely dependent upon the boiling properties of the constituents to be removed (for example organic solvents) and upon the waste-air outlet temperature reached in the condenser. Disadvantages of this process include its poor thermal efficiency and, above all, the content of noncondensible or non-condensed polluting constituents in the proportion of waste gas released into the atmosphere.
In another known process for the heat treatment of waste air ("Textilprasix International" 1976, page 281), the pollutant-containing waste air from a textile treatment unit is delivered to a combustion chamber and is heated to a reaction temperature of around 750.degree. C. by admixture with the waste gases from an oil or gas burner. In this way, the hydrocarbons burn to form steam and carbon dioxide. In this connection, attempts have already been made to recover a certain amount of heat, for example by heating the impure waste air with the purified waste air in a heat exchanger preceding the combustion unit. However, close examination of the heat balance and the necessary investment costs has shown that the technical outlay involved in this known thermal waste-air treatment process is still uneconomically high.
Accordingly, the object of present invention is to obviate the disadvantages of conventional processes by providing a process of the type referred to above which considerably reduces the amount of waste gas released into the atmosphere in a particularly economical manner.